1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a backlight assembly for a liquid crystal display (LCD) device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for reducing the numbers of the transformers and the driving circuits for the transformers in the backlight assembly for the LCD device.
2. Discussion of the Related Art
Until recently, cathode-ray tubes (CRTs) have been typically used in display devices. Presently, many efforts are being made to study and develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays (FED), and electro-luminescence displays (ELDs), as a substitute for CRTs. In particular, these types of flat panel displays have been driven as an active matrix type in which a plurality of pixels arranged in a matrix form are driven by a plurality of thin film transistors therein. Among the active matrix types of flat panel displays, liquid crystal display (LCD) devices have been widely used as monitors for notebook computers and desktop computers because of their high resolution, ability to display colors and superiority in displaying moving images.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes into the direction of the induced electric field in accordance with the intensity of the induced electric field into the direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field. Since the LCD devices are non-luminous type display devices, the LCD devices require a backlight assembly for supplying light.
The backlight assembly is categorized as an edge type backlight assembly or a direct type backlight assembly according to the arrangement thereof. A lamp of the edge type backlight assembly is disposed at side portions of the LCD device, while a lamp of the direct type backlight assembly is disposed at the rear of the LCD device. As size of the LCD device gets larger, a higher luminance backlight is required. The direct type backlight assembly is more widely used because of its ability to provide higher luminance.
FIG. 1A is a schematic plan view of a direct type backlight assembly of the LCD device according to the related art. As shown in FIG. 1A, the backlight assembly includes a plurality of U-shaped lamps 110, a lamp housing 120 accommodating the U-shaped lamps 110 therein, and an inverter 100. The inverter 100 includes a conversion circuit (not shown) converting a direct current (DC) voltage to an alternating current (AC) voltage, a plurality of transformers 130 boosting the converted AC voltage, and a plurality of output connectors 140 supplying the boosted AC voltage to the U-shaped lamps 110. Each U-shaped lamp 110 is connected to the two corresponding transformers 130 through the corresponding connector 140.
FIG. 1B is a waveform diagram of lamp driving voltages input to the U-shaped lamp of FIG. 1A. As shown in FIG. 1B, first and second lamp driving voltages “WF1” and “WF2” are alternately output from the transformers 130 (shown in FIG. 1A) and thus are supplied respectively to the two electrodes of each U-shaped lamp 110 (shown in FIG. 1A). The first and second lamp driving voltages have inverse phases to each other, i.e., 180 degrees, and thus a center portion of each U-shaped lamp 110 acts as a virtual ground 111. Since the adjacent lamp driving voltages “WF1” and “WF2” have inverse phases, the wave noise of the LCD device is improved.
FIG. 2 is a schematic circuit diagram of a driving circuit for a transformer according to the related art. As shown in FIG. 2, the driving circuit includes a switching portion 230, a controlling portion 220, a voltage detecting portion 210 and a feed-back line 240. The driving circuit is connected to each transformer 250. The voltage detecting portion 210 outputs a feed-back signal to the controlling portion 220 through the feed-back line 240. The switching portion 230 selects voltages output from a voltage source in response to a controlling signal of the controlling portion 220. The controlling portion 220 controls the switching portion 230 in response to a feed-back signal of the voltage detecting portion 210.
The transformer 250 includes a primary winding 251 and a secondary winding 252. Both terminals of the primary winding 251 are connected to the switching portion 230. One terminal of the secondary winding 252 is connected to an U-shaped lamp 200, and the other terminal of the secondary winding 252 is connected to the voltage detecting portion 210. Accordingly, the voltage detecting portion 210 outputs the feed-back signal corresponding to a voltage of the other terminal of the secondary winding 252.
The above related art backlight assembly uses two transformer to drive each U-shaped lamp. Therefore, as the number of the U-shaped lamps increases, the numbers of the transformers and the driving circuits for the transformers also increase. The increasing numbers of transformers and the driving circuits for the transformers take up more space and add more weight.